Sensor with holographic multiplexed image display

ABSTRACT

A holographic sensor comprising a thin film polymer matrix that undergoes a change in response to a substance to be sensed, the matrix containing within its volume a set of two or more holographic recordings, each recording providing a holographic image when the sensor is illuminated, wherein the presence or appearance of each image is visible to the eye as a function of the response of the sensor to the substance to be sensed. The images provide the dynamic range of the sensor. Such a sensor can be used to provide a visible image that changes or appears to the eye in response to an analyte.

FIELD OF THE INVENTION

[0001] The present invention relates generally to sensors and moreparticularly to a holographic multiplexed image sensor.

BACKGROUND OF THE INVENTION

[0002] Chemical sensors and biosensors in the form of volume hologramsmade in specially made polymer layers are known. WO-A-95/26499 disclosesa sensor which comprises a reflection hologram made in a thin film ofpolymeric material where the polymer interacts with a substance to bedetected so as to alter the optical properties of the hologram, therebyproviding a means for detecting or quantifying that substance. Moregenerally, this reference and also WO-A-99/64308 disclose the concept ofa volume hologram sensor which provides a measurable or observableoptical change.

[0003] Within the art of holography, multiple holographic images andmethods for creating them in a single holographic recording material areknown. U.S. Pat. No. 4,509,818 discloses a method of making athree-dimensional holographic multiplexed image from a series oftwo-dimensional images. U.S. Pat. No. 5,103,325 discloses a method ofholographically recording a series of two-dimensional images such thatthe viewed holographic images are observed separately and distinctlyfrom each other. U.S. Pat. No. 5,734,485 discloses a method of producingthree-dimensional still or moving scene holograms including recordingsof computer-generated scenes.

[0004] These known systems produce sets of holographic images which aremultiplexed in a degree-of-freedom which is only spatial, where theimages are intended to be viewable by an observer as anartificially-produced three-dimensional image or as a set of imagesseparated in space over a corresponding set of angles of view. Theoptical properties of the material in which these holograms are made areintended to be invariant in time and they are not intended to be alteredchemically when functioning normally.

SUMMARY OF THE INVENTION

[0005] An object behind the present invention is to provide a volumehologram sensor which provides a multiplicity of holographic images,where the set of images is multiplexed in the degree-of-freedom which isthe dynamic detection range of the sensor, where each image, whenvisible, represents a finite region of the dynamic detection range.

[0006] According to a first aspect of the present invention, aholographic sensor comprises one or more films each containing withinits volume a set of two or more holographic recordings, each recordingproviding a reflected holographic image when the sensor is illuminatedby light and where each image is visible to the eye as an indicator thatthe sensor is showing a response to a predetermined range ofconcentration of a substance or group of substances to be sensed. Moreparticularly, the presence or appearance of each image is visible to theeye as a function of the response of the sensor to a substance to besensed; that response may involve the appearance or disappearance, or achange in, a visible image.

[0007] Typically, each image in the set of images has a reflectionspectrum characterised by its location in the invisible or visiblespectrum of light. The location in the spectrum may be unique to eachimage, such that the images are separable by wavelength-selective meansand are therefore wavelength-multiplexed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 shows an example of the multiple spectral peaks of amultiplexed reflection hologram, typical of those exhibited by amultiplexed holographic sensor.

[0009]FIG. 2 shows another example of the multiple spectral peaks of amultiplexed reflection hologram typical of those exhibited by amultiplexed holographic sensor.

[0010]FIGS. 3a and 3 b are each schematic drawings of a holographicsensor showing changing pictorial images.

[0011]FIGS. 4a to 4 d are each schematic drawings of a holographicsensor illustrating a changing image in correspondence with the amountof substance detected.

[0012]FIG. 5 is a schematic drawing of multiple features of athree-dimensional image.

[0013]FIGS. 6a to 6 e are each schematic representations of aholographic sensor with changing numerical images.

[0014]FIGS. 7a to 7 e are each schematic representations of aholographic sensor with calibrated numerical images.

[0015]FIGS. 8a to 8 e are each schematic representations of aholographic sensor with changing alphabetical images.

[0016]FIGS. 9a and 9 b are each schematic representations of aholographic sensor illustrating changing images in the form of messagesrelating to the application of the sensor.

[0017]FIGS. 10a to 10 c are each schematic representations of aholographic sensor illustrating changing images in the form of messagesrelating to the amount of substance or substances being detected.

[0018]FIG. 11 is a schematic representation of a holographic sensorillustrating a changing image in the form of a moving indicator againsta fixed scale.

[0019]FIG. 12 is a schematic representation of a holographic sensorillustrating a changing image in the form of an indicator moving in thedepth of the image against an image of a scale located in the depth ofthe image.

[0020]FIG. 13 is a schematic of an optical layout which can be used toexpose a photosensitive holographic film or plate to multiple imageswith the purpose of making a wavelength multiplexed holographic sensor.

DESCRIPTION OF THE INVENTION

[0021] A sensor of the invention may be constructed and used in themanner generally described in WO-A-95/26499 or WO-A-99/63408. Thecontents of these publications, and other documents referred to herein,are incorporated by reference. Thus, for example, the matrix in whichthe holographic images are formed may be a chemically sensitivepolymeric film, or it may comprise a plurality of films that aregenerally parallel (adjacent or separated by another type of layer). Insuch an arrangement, each film may provide its own dynamic range, andeach film may be designed to detect or measure a specific substance.Each film may present one image or a sub-set of images with its ownplace in the dynamic range of the sensor; the dynamic range is createdby having a plurality of films which provide a plurality or set ofimages.

[0022] More particularly, a sensor of the invention can be in the formof a polymer film or multiple films coated or otherwise disposed onto atransparent or opaque, flexible, semi-rigid or rigid substrate such asglass, plastic, paper or metal. The substrate can be printed, engravedor otherwise marked with a pattern or alpha-numerical markings so as toprovide a reference to the holographic images.

[0023] The sensor can, alternatively, be provided in or onto a materialwhich is component of or constitutes a device such as contact lens,spectacle lens, optical window into a reaction vessel, instrumentdisplay window, domestic window, visual display device or any componentwhere an ambient substance is to be monitored or detected.

[0024] The sensor can, alternatively, be provided in or onto a materialwhich is a component of or constitutes an item of clothing so as toconfer the ability to monitor or detect ambient substances orphysiological substances related to the wearer of the clothing.

[0025] The invention can be in the form of multiple layers ofholographic polymer films which are interleaved with other types oflayers acting as transport media for substances to be detected ormonitored or other components of a sample.

[0026] Illumination of the hologram(s) by ambient artificial or naturallight can be directly onto the plane surfaces or, alternatively, can beprovided by illuminating the polymer films along their edges, where theholograms are commonly known as “edge-lit” holograms.

[0027] A polymer film which is a sensitive element of the invention maybe directly sensitive to an ambient substance or it may be sensitive tothe product of a reaction or interaction between the ambient substanceand one or more other ambient substances or substances which areprovided specifically as components of the holographic sensor assembly.Such a film may be described herein as chemically sensitive, but this isfor the purpose of illustration only.

[0028] Any of a variety of substances or analytes may be detected bymeans of the invention, including but not limited to those discussed inthe prior art; reference herein to “a substance” includes the use of twoor more such substances. Examples of analytes are water, organicliquids, ions, haptens, nucleotides, cells, aldehydes, enzymes,proteins, gases, metabolites, viruses, bacteria, fungi and yeasts. Theanalyte or a carrier medium may interact with the holographic matrix. Inparticular examples, the analyte is in liquid, e.g. an enzyme or ethanolin water, or water in an organic solvent.

[0029] In a preferred embodiment of the invention, each image from theset of the pictorial images that can be viewed depicts subject matterwhich is relevant to the sensor application. Each image may depictsubject matter which is relevant to the response status indicated by thesensor.

[0030] The image may change from one picture to another in relation tothe concentration of one or more substances to be detected by thesensor. The change in the pictorial image may be restricted to one ormore parts of the image. A change in the pictorial image which isrestricted to part of the image may be due to a response to a specificsubstance to be detected, such that a change in another part of theimage is due to a response to another specific substance to be detected.Each part of the image which may be changed may be located anywhere inthe three dimensions of the holographic image.

[0031] In another preferred embodiment of the invention, the set ofimages shows a sequence of numerical information which appears in asequence corresponding to the concentration of one or more substancesdetected by the sensor. Preferably, the response of the sensor iscalibrated so that the numerical images show numerical quantities whichcorrespond directly with the concentration of a substance detected bythe sensor.

[0032] In another preferred embodiment of the invention, the set ofimages shows a sequence of alphabetical information which appears in asequence corresponding to the concentration of a substance detected bythe sensor. Preferably, the alphabetical information is in the form ofmessages which are relevant to the sensor application. The response ofthe sensor may be calibrated so that the alphabetical information is inthe form of messages which correspond directly with the concentration ofa substance detected by the sensor.

[0033] In another preferred embodiment of the invention, each image ofthe set of images comprises an indicating feature which has a specificlocation, in the space of the image, corresponding to the concentrationof a substance detected by the sensor. This is an example of a virtualinstrument.

[0034] Preferably, the image or indicating feature is a shape.Alternatively, the indicating feature is a picture or isalpha-numerical.

[0035] The spatial degree of freedom of the location of the indicatingfeature may be parallel to the plane of the polymer film. Alternatively,the spatial degree of freedom of the location of the indicating featureis not parallel to the plane of the polymer film but is, instead, in thedepth of the image which is an optional characteristic of a holographicimage.

[0036] Preferably, the location of the indicating feature in either caseis marked with reference to a visible scale. The visible scale may beprovided as a holographic image provided by a hologram recorded in thesame polymer layer as that which provides the indicating feature.Alternatively, the visible scale may be created by a holographic imageprovided by a different polymer layer from that which provides theindicating feature.

[0037] The visible scale may be incorporated with the polymer layer byphotographic means. Alternatively, the visible scale may be printed ontothe surface of the holographic element, or it may be printed onto asurface which is located adjacent to but separate from the holographicelement.

[0038] Preferably, the visible scale which is provided as a holographicimage is invariant with the concentration of the substance detected bythe sensor.

[0039] A holographic sensor can provide any combination of pictorial,alphabetical, numerical or spatially-indicating means of displaying theholographic response. Further, an array of holographic sensors may beprovided, each providing any combination of the above pictorial,alphabetical, numerical or spatially-indicating means of displaying theholographic response to a multiplicity of substances to be detected ormultiplicity of groups of substances to be detected.

[0040] Preferably, each element of an array of holographic sensors has aunique response characteristic to the substances to be detected.

[0041] The visible display provided by an array of holographic sensorsmay present an overall pattern which corresponds to the relativeconcentrations of substances to be detected. The pattern displayed by anarray of holographic sensors may be pictorial, numerical oralphabetical. An alphabetical pattern displayed by an array ofholographic sensors may represent a message which is relevant to therelative concentrations of substances to be detected.

[0042] In any of the above cases, the discrimination of any one visibleimage from its neighbours in a set of images presented by theholographic sensor can be provided by creating a significant separationin the peak reflected wavelength provided by each image from that of itsneighbours.

[0043] The discrimination of any one visible image from its neighboursin a sequence of images presented by the holographic sensor may beimproved by providing a colour transmission filter located between thelight source and a chemically-sensitive polymer film containing theholographic recordings, or between the eye used to view the holographicimage and the film, or immediately adjacent to the film but between thefilm and the eye.

[0044] The colour transmission filter may be an integral feature of thematerial to which a chemically-sensitive polymer film is attached.Alternatively, the colour transmission filter may be an integral featureof the chemically-sensitive polymer film. In any of the aboveholographic sensors, a colour transmission filter increases the numberof multiplexed images for any given dynamic range of response of thesensor, by permitting each image to be closer in peak wavelength to thatof its immediate spectral neighbour.

[0045] According to a further aspect of the present invention, a methodfor creating a holographic sensor which has a multiplicity ofwavelength-multiplexed images of one or more types chosen frompictorial, numerical, alphabetical, spatially-variant or array types,comprises exposing a polymer film, having already been photosensitised,to a sequence of holographic exposures over the course of a transitionof the film from one state of swelling to another.

[0046] Each image of the set of images has a characteristic reflectionspectrum which may have a peak wavelength which is different from thatof other images in the set.

[0047] By way of example, the initial state of swelling may be set byplacing the polymer film, before exposure, in a solution having aspecific pH or ionic strength. Then the polymer film is immersed in asolution with a different specific pH or ionic strength, respectively,so that the film undergoes a transition of swelling or contraction,depending on its response.

[0048] Alternatively, the initial state of swelling is set by placingthe polymer film, before exposure, in an immediate environment having aspecific relative humidity. Then the relative humidity is altered sothat the film undergoes a transition of swelling or contraction,depending on its response to relative humidity.

[0049] An alternative method for creating a holographic sensor which hasa multiplicity of wavelength-multiplexed images of one or more typeschosen from pictorial, numerical, alphabetical, spatially-variant orarray types is to expose the photosensitive polymer film to each imageso that the angle between the object and reference beams used to createthe holographic recording is unique to that particular image.

[0050] A preferred method for exposing the photosensitive polymer filmto a set of images is to expose it to a timed sequence of images of atransmission object where the transmission object is an optical devicewhich is commonly known as a spatial light modulator and is controlledby an electronic signal source, e.g. a computer or a video camera.Preferably, the form of the object represented by the spatial lightmodulator is chosen from pictorial, numerical, alphabetical,spatially-variant or array types.

[0051] Preferably, the image provided by the spatial light modulator iscontrolled so as to have variable spatial features during the transitionof swelling or contraction, so as to provide a means of providing aholographic sensor which has a spatially-variant response to a range ofconcentrations of a substance to be detected.

[0052] The present invention will now be described by way of exampleonly with reference to the accompanying drawings. These drawingsillustrate the changing display of two or more holographic images inresponse to a substance or group of substances to be detected by aholographic sensor.

[0053] In any form of the invention, there exist two or more reflectedholographic images, each with a colour characterised by a narrow bandspectrum having a peak wavelength. A peak wavelength arises fromconstructive interference between components of light reflected anddiffracted from a periodic structure such as a holographic structurewhich is composed of a periodic distribution of complex refractive indexcontained within a thin film of holographic material which is commonly apolymer or similar matrix. In holography, such a periodic distributionof refractive index is commonly known as a distribution of fringes. Thepeak wavelength is defined mathematically by the Bragg equation which is

λ _(pk){x,y,z}=2.n{x,y,z}.Λ{x,y,z}.cos(θ{x,y,z})

[0054] where n is an average index of refraction of the polymer film ata particular location defined generally by the co-ordinates x, y and zin the film, Λ is the local spacing between adjacent fringes and θ isthe angle of illumination of light which is incident on the fringes atthat location in the film.

[0055]FIG. 1 shows a reflected intensity spectrum with a wavelength axis4 showing three spectral peaks 1, 2 and 3 at one particular state in thedynamic range of the sensor. At this state, the only visible image isthat characterised by the peak 2, situated in the region 5 of thespectrum which is normally visible to the eye, bordered by theultra-violet end of the spectrum 6 and by the infra-red end of thespectrum 7. If the polymer film in which the sensor hologram is madeswells during operation of the sensor then the characteristic peakwavelengths of the peaks 1, 2 and 3 all shift to longer wavelengths suchthat the image characterised by peak 1 originally invisible in theultra-violet end of the spectrum becomes visible in a new spectrallocation 8. Similarly, the previously visible image characterised by thespectral peak 2 becomes invisible in the infra-red part of the spectrum,at a spectral location 9. Similarly, a response of the holographicsensor which is a contraction of the polymer film in which the sensorhologram is made is characterised by a shift of the peaks 1, 2 and 3 toshorter wavelengths.

[0056] In an alternative form of the invention, more spectral peaks perregion of the spectrum can be provided whilst maintaining discriminationbetween adjacent images. FIG. 2 shows a restriction of the region 5 ofthe spectrum which is available to be seen by eye or other detector to anarrower region 11 bounded by a lower end 30 set in this example by along-wavelength pass filter and an upper end 7 at the upper end of thenormally visible part of the spectrum 5. In general, a means ofrestricting the visible spectrum is not confined to a long wavelengthpass edge filter but can be chosen from long wavelength pass filter,short wavelength pass filter, band-pass filter or any other opticaldevice which restricts the detectable part of the whole spectrum. FIG. 2shows a reflected intensity spectrum with a wavelength axis 4 showingfour spectral peaks 1, 2, 13 and 14 at one particular state in thedynamic range of the sensor. At this state, the only visible image isthat characterised by the peak 2, situated in the narrower region 11 ofthe spectrum which is visible to the eye. If the polymer film in whichthe sensor hologram is made swells during operation of the sensor thenthe characteristic peak wavelengths of the peaks 1, 2, 13 and 14 allshift to longer wavelengths such that the image characterised by peak 13originally invisible in the ultra-violet end of the spectrum becomesvisible in a new spectral location 16. As the new image characterised bythe spectral peak 16 appears the original visible image characterised bythe spectral peak 2 becomes invisible as it moves to a new spectrallocation 17. As further swelling occurs the image characterised by thepeak 1 becomes visible in the spectral location 16, or some such similarlocation in the confined visible region 11. One purpose of providingmore spectral peaks per region of the spectrum is to allow a visiblechange in image to occur in response to a small swelling or contractionof the polymer film in which the holographic images are recorded.Another purpose of providing more spectral peaks per region of thespectrum is to provide a greater number of images throughout the dynamicrange of the holographic sensor.

[0057] A preferred form of the invention is illustrated in FIG. 3a whichshows a schematic representing a holographic image 31 of a car providedby a holographic recording in a piece of holographic material 30. Inthis particular example, the car represents a purpose for which aholographic sensor may be designed, that of detecting the excessivepresence of alcohol in the breath of an individual person. One way inwhich the device represented in FIG. 3a may be used is to have apreviously invisible image which becomes visible when saturated withmoisture from the breath. In another way of using the device, the imagesuch as that illustrated 31, could be always visible if provided in astate of saturation. The detection of excess alcohol in the breath isindicated by the change of the image 31 in FIG. 3a to another image 32in FIG. 3b where the image illustrates pictorially that the testedperson should not drive. The illustrations are given by way of exampleonly and do not preclude the use of other pictorial images to conveyother messages and instructions for the purpose of the use described orfor any other application which uses pictorial information to illustratethe relative response of the sensor before and after use.

[0058] Another preferred form of the invention is illustratedschematically in FIG. 4a which shows a holographic material 30providing, under illumination, an image of a shape 43 with a part 44which is differentiated from the scale 43 by having a differentappearance by way of colour, shape or pattern. The response of thesensor is indicated by the change in the image segment 44 to that 45shown in FIG. 4b, illustrating an increase in the presence of asubstance which is detected by the sensor by occupying a greater part ofthe image 43. Sequential response to greater amounts of a substancedetected is indicated by progressive changes in parts 44 to 47 of theimage 43, illustrated in FIGS. 4a to 4 d. In this example, the spatialchanges of the image or parts of the image are key features of thispreferred form of the invention. An example of a particular applicationwhich utilises these essential features of the invention is as a medicaldiagnostic device which shows an image of a stylised form of the humanbody where a part of the image appears to be illuminated to indicate abiochemical, metabolic or pathological condition relating to therelevant part of the body.

[0059]FIG. 4 illustrates a set of images where features of the imagesare located in a plane in space. The essential features of the inventionare not limited to planar images but can, alternatively, be employed inthree-dimensional holographic images. In another preferred form of theinvention, the spatial changes of the holographic image or parts of theimage are located in the three-dimensional space of the image. FIG. 5shows a sensor made in a holographic material 30 which provides an imagein three dimensions indicated by the axes 54 in x, y and z and havingfeatures 51, 52 and 53. The features 51, 52 and 53 can be made to appearor disappear or change in appearance by way of colour, shape or patternas the visible means of observing the operation of the sensor.

[0060] In any holographic sensor where an image or part of an image ismade to change or become visible or invisible, the image or part of animage can have numerical form, as illustrated schematically in FIG. 6a.Numerical images 60-65 shown in FIGS. 6a to 6 e illustrate a response inrelation to the concentration of a substance or group of substances tobe detected by the sensor. Alternatively, the numerical response of aholographic sensor can be calibrated to the concentration of a substanceor group of substances to be detected, as illustrated in FIGS. 7a to 7e, by images 71-75.

[0061] In any holographic sensor where an image or part of an image ismade to change or become visible or invisible, the image or part of animage can have alphabetical form, as illustrated schematically in FIG.8a. Alphabetical images 81-85 shown in FIGS. 8a to 8 e illustrate aresponse in relation to the concentration of a substance or group ofsubstances to be detected by the sensor.

[0062] In any holographic sensor, the images can optionally show acombination of numerical or alphabetical information relating to theapplication for which the sensor is intended.

[0063] In any holographic sensor which presents alphabeticalinformation, the message which is provided can be related to theapplication for which the sensor is intended. An example of aholographic sensor for breath alcohol is illustrated in the schematic ofFIG. 9a which shows a message 91 indicating that the measured level iswithin bounds accepted by predetermined rules. The schematic of FIG. 9billustrates an example of a message 92 which indicates that the measuredlevel falls outside bounds accepted by predetermined rules.

[0064] In any holographic sensor which provides alphabeticalinformation, the message which is provided can be related to theconcentration of substance or group of substances to be measured. FIG.10a shows an alphabetical image 101 which indicates a low detected levelof substance or group of substances. FIGS. 10b and 10 c indicate,respectively, normal and high levels, by images 102 and 103.Alternatively, the messages provided can be an indicator as to thecourse of action to be followed as a consequence of carrying out thetest provided by the holographic sensor.

[0065] The presentation of simple messages in the fashion provided byholographic sensor devices provides an unambiguous and easily understoodresult and is particularly suitable for rapid tests or use by unskilledpeople in a variety of healthcare, consumer or clinical applicationsthough other applications areas are included.

[0066] In another preferred form of the invention, illustratedschematically in FIG. 11, each of the multiplexed holographic images isin the form of a pointing indicator. A series of such indicators ismultiplexed according to methods described above such that, preferably,only one is visible at any one response state of the sensor. In theexample shown in FIG. 11, just two of the indicator images 112 and 113are shown, though a series of images separated spatially along thedirection 114 provides a sequence related to the concentration of asubstance or group of substances to be quantified. The pointingindicators 112 and 114 and others not shown in the diagram refer to ascale 111 which can be pictorial or numerical. A numerical scale 111provides a means of quantifying the response of the sensor. The scale111 can be chosen from the following types: printed adjacent to theholographic material, printed onto the holographic material, printed ona separate material under the holographic material, photographicallycreated separate to the holographic material, photographically createdwithin the holographic material, holographically created within theholographic material, holographically created in a separate holographicmaterial from that which serves as the sensor material, though the listis not exclusive.

[0067] In another preferred form of the invention, illustratedschematically in FIG. 12, each of the multiplexed images is in the formof a pointing indicator which appear to be arranged in three dimensions,out of the plane of the holographic material 30. The characteristicdepth which is optionally provided by holographic images is utilised inthis form of holographic sensor. A series of such indicators ismultiplexed according to methods described above such that, preferably,only one is visible at any one response state of the sensor. In theexample shown in FIG. 12, just two of the indicator images 122 and 123are shown, though a series of images separated spatially along thedirection 124 in three spatial dimensions provides a sequence related tothe concentration of a substance or group of substances to bequantified. The pointing indicators 122 and 124 and others (not shown)refer to a scale 121 which can be pictorial or numerical. A numericalscale 121 provides a means of quantifying the response of the sensor.The scale 121 is preferably itself a holographic image which is alignedwith the sequence of multiplexed pointing indicator images though it canbe chosen from the following types: printed onto the holographicmaterial, printed on a separate material under the holographic material,photographically created separate to the holographic material,photographically created within the holographic material,holographically created within the holographic material, holographicallycreated in a separate holographic material from that which serves as thesensor material, though the list is not exclusive. Some benefits ofusing three-dimensional holographic images in a holographic sensor arethat the area of the holographic material can be reduced, allowing testsample volumes to be reduced, manufacturing cost to be reduced and spaceto be saved.

[0068] A preferred method for constructing the multiplexed images forthe purpose of providing a holographic sensor is to use a two-beamimage-hologram process such as that illustrated by way of example in theschematic of FIG. 13. A laser beam 131 is split into two beams 132 and133 by a beam-splitter 134. One of the beams 132 is directed by a mirror135 onto a transparent object 136 via an optional light diffuser 140.Preferably, the transparent object 136 is a spatial light modulatorwhich is a video display device which provides an image under computercontrol. Alternatively, the transparent object 136 can be a photographictransparency. A benefit of using a computer-controlled spatial lightmodulator is that the transparent objects it provides as images to berecorded holographically can be rapidly changed in order to create thesequence of holographic images. The illuminated transparent object 136is located at the object plane of an imaging system 137 which is a setof one or more lenses which provides an image of the object 136 at animage plane where a holographic recording material 138 is situated. Thesecond laser beam 133 is directed, in this example, by a mirror 139 ontothe holographic recording material 138 and thus acts as a reference beam(in holographic terminology). The image and the reference beams combineto produce an interference pattern in the holographic recording material138 in such a way as to allow it to be retained by the material. Twoexamples of methods of recording a holographic interference pattern areby further chemical processing, if a silver-based recording material, orby using a photo-polymer material and appropriate laser wavelength. Anessential feature of this aspect of the invention is that the state ofswelling of the holographic recording material 138 is controllable bysome means chosen from pH, ion concentration, humidity, water activityor any other means of altering the thickness of the holographicrecording material. At each state of swelling of the material adifferent holographic image is created by the means described until acomplete set has been recorded as a set of multiplexed images whichdisplay the response of a holographic sensor in the formats describedabove.

1. A holographic sensor which comprises a thin film polymer matrix thatundergoes a change in response to a substance to be sensed, the matrixcontaining within its volume a set of two or more holographicrecordings, each recording providing a holographic image when the sensoris illuminated, wherein the presence or appearance of each image isvisible to the eye as a function of the response of the sensor to thesubstance to be sensed, and the images provide the dynamic range of thesensor.
 2. A sensor according to claim 1, wherein each image has aunique location in the visible or invisible spectrum.
 3. A sensoraccording to claim 1 or claim 2, wherein each image is pictorial,numerical and/or alphabetical.
 4. A sensor according to any precedingclaim, which provides a visible image in the presence of the substance.5. A sensor according to claim 4, wherein the visible image isrepresentative of the substance or a scenario in which the substance isfound or the sensor is used.
 6. A sensor according to claim 5, whereinthe visible image is numerical.
 7. A sensor according to claim 5,wherein the visible image is alphabetical and provides a messagecorresponding to the substance or a scenario in which the substance isfound or the sensor is used.
 8. A sensor according to claim 5, whereinthe visible image has a location, in the space of the image,corresponding to or representative of the substance or a scenario inwhich the substance is found or the sensor is used.
 9. A sensoraccording to any of claims 4 to 8, wherein the visible image iscalibrated with respect to the quantity of the substance.
 10. A sensoraccording to any of claims 4 to 9, which provides another visible imagein the absence of the substance.
 11. A sensor according to any precedingclaim, which additionally comprises a visible scale.
 12. A sensoraccording to claim 11, wherein the scale is invariant with respect tothe concentration of the substance.
 13. A sensor according to anypreceding claim, which provides multiple images within one film.
 14. Asensor according to any preceding claim, which provides at least threeimages.
 15. A sensor according to any preceding claim, which comprises aplurality of films each providing its own dynamic range within thedynamic range of the sensor.
 16. A sensor according to any precedingclaim, which comprises a plurality of films each responding to adifferent substance.
 17. A sensor according to any preceding claim,which additionally comprises a colour transmission filter.
 18. A sensoraccording to any preceding claim, where the image is formed from a setof visible pixels, the pattern of pixels being indicative orrepresentative of the substance or a scenario in which the substance isfound or the sensor is used.
 19. Apparatus comprising a sensor accordingto any preceding claim and an illumination source.